The metabolism of vitamin D3 has been greatly elucidated in recent years, with the recognition that it must undergo 25-hydroxylation in the liver and 1-hydroxylation in the kidney before being maximally active at its various target tissues (principally intestine and bone). Although previous studies have successfully clarified much about these steps in experimental animals, little is known about quantitative aspects of the sequence, especially in man, except by observation of responses to administration of various metabolites and congeners. It is proposed to utilize stochastic and/or compartmental analysis after injection of 3H-25-hydroxy-vitamin D3 to quantitate in man several important aspects of vitamin D3 metabolism: rates of synthesis and disposal of 25-hydroxy-vitamin D3, 1,25-dihydroxy-vitamin D3, and perhaps other metabolites of interest, total body pool size of each metabolite, intestinal absorption and excretion, and effects of various interactive factors on these parameters. Studies will be performed in normal volunteers and in patients with a variety of diseases that have been characterized by vitamin D "resistance" or "sensitivity", such as cirrhosis, malabsorption, osteoporosis, sarcoidosis, etc. An important development permitting us to proceed with these studies is our recent success in raising an antibody that seems to be specific for the general class of vitamin D metabolities possessing a 25-hydroxyl group. Coupled with high pressure liquid chromatography, we shall be able to separate and quantitate each metabolite. It is predicted that data obtained by this method will provide a rational basis for therapeutic uses of vitamin D and its metabolites.